JP2009535628A - Piezoelectric measuring element with lateral effect and sensor including said measuring element - Google Patents

Abstract

  The invention relates to a piezoelectric measuring element (1) comprising at least one bar (2) with a lateral effect that can use two-conductor technology. The present invention provides one or more lateral effects such that the metallized surface (5), electrode (6) and line (15) of the measuring element (1) have the same capacitance with respect to the housing (13). Based on the idea implemented by the piezoelectric rod (2). This is achieved, for example, by a completely symmetrical design of the pole (22) or by providing a capacitor from one pole (22) to the housing (13) corresponding to the capacitance difference of the poles (22) with respect to the environment. Is done.

Description

The invention relates to a piezoelectric measuring element according to the features of claim 1 and to a sensor comprising said measuring element.

  Piezoelectric measurement elements are used in several applications, such as pressure, force, elongation or acceleration sensors. During the measurement, the body made of piezoelectric material is subjected to mechanical strain, and charges are generated on specific surfaces of the body. This charge is collected and guided to the cable connection by the electrode. Eventually, the cable transfers the charge to an amplifier that may be a preamplifier.

  Basically, there are two different possibilities for the transfer of charge to the amplifier: the use of coaxial cables or the use of two conductor cables. When a coaxial cable is used, measurement data is transmitted from the first electrode to the inner conductor, while the other electrode is on the main potential and can be connected to a shield that protects the measurement line from external interference fields. Further, it is possible to provide the second shield with a triaxial cable. In contrast, when two conductor cables are used, the two electrodes are connected by two conductors of the cable that can shield themselves in whole or double. In the amplifier, the charge difference between the two leads is evaluated. Since the lead wires are exposed to the same interference electric field, the change in charge difference does not occur from the interference electric field.

  An example application using two-conductor technology is a measuring element having one or more piezoelectric plates with a longitudinal effect, for example. The longitudinal effect means that electric charges are generated on the surface to which force is applied. These surfaces are most often metallized to collect charge. Each metallized surface of the same polarity is connected to one electrode, which is applied to one of the two conductors. Since the metallized surface is loaded, good contact between adjacent surfaces is always guaranteed.

  Applications using coaxial technology also include a measuring element having one or more piezoelectric plates with longitudinal effects, each electrode connected to an inner conductor and a shield. On the contrary, the use of side effect bars constructed from piezoelectric material by coaxial technology is also known. The lateral effect means that the surface on which charge is generated under load is located perpendicular to the surface to which load is applied. EP1283552 describes an application of this type including one, and WO2005 / 026678 describes an application including several bar crystals of lateral effects. In these cases, the charge collecting electrode is electrically connected to the metallized surface on the force application surface to ensure a good contact to collect the charge.

  The advantage of the lateral effect piezoelectric rod over the longitudinal effect plate lies in its sensitivity and cost performance ratio. In order to obtain the same sensitivity achievable with thin rods of plates, a laminate of multiple plates must be used, which is much more expensive and much higher than the rods. Several or thin bars can be used to further increase the stability and sensitivity of the measuring element including the bars and as described in WO 2005/026678. In this application, the electrodes are electrically connected to the metallized surfaces on the two force-applying surfaces of the piezoelectric rod, which gives a good contact.

  Another application using several rods is also known from FIG. 2 of EP1283552. Here, three rods are arranged in a triangle, and inside the triangle, the spring receives charge as the first electrode, while the spring simultaneously presses the outer wall of the crystal against the metal wall, which is the second electrode.

  The advantage of two-conductor technology compared to coaxial technology is ground insulation. Each measuring element in two conductor technology is necessarily grounded, while measuring elements using coaxial technology often have electrodes at the main potential. Grounding insulation can only be obtained by delicate assembly using the measuring element's insulating ring and shield. This is essential, for example, in the case of internal combustion engines with long signal leads in order to compensate for the voltage at the ground connection, so as to prevent interference by so-called ground loops. Note that the use of sensors in internal combustion engines requires temperature stability at least at 300 ° C. This requires specific elements that can withstand these temperatures.

  It is not possible to use the two conductor technique with known lateral effect bars, because the structure causes the individual electrodes to be subject to different interfering electric fields and thus bias the measurement signal.

  The object of the present invention is to suggest a piezoelectric measuring element comprising at least one bar of lateral effect that can be used in two-conductor technology.

  This object is achieved by the features of the independent claims.

  The idea lying under the present invention is realized with one or more piezoelectric rods due to the lateral effect, so that the pole composed of the metallized surface of the measuring element, the electrodes and the line has the same electric capacity with respect to the housing. That is. This can be achieved, for example, by a completely symmetrical design of the poles or by providing a capacitor in the housing from one pole that corresponds to the capacitance difference between the poles relative to the environment.

  FIG. 1 shows in cross-section a ground-insulated measuring element of an internal combustion engine according to the prior art. This includes a bar 2 constructed from a lateral effect piezoelectric material. The bar 2 includes two opposing surfaces 3a and 3b which are surfaces to which a force to be measured is applied, and opposing surfaces 4a and 4b provided with metallized surfaces 5a and 5b for collecting surface charges. Each of the metallized surfaces 5a, 5b extends to the electrodes 6a, 6b on each of the front surfaces 3a, 3b. The rod 2 is located in the internal space of the housing 7 which is also a tension sleeve that applies pretension to the rod 2. A cover 9 encloses a housing 7 that houses the rod 2. The bar 2 can have any suitable area, in particular a rectangle, ellipse, concave or convex area or a combination thereof.

  The sensitivity of the side effect square bar is proportional to the ratio of the length of the bar defined by the distance between the load receiving surfaces 3a, 3b and its thickness defined by the distance between the charge collection surfaces 4a, 4b. Therefore, the sensitivity increases proportionally as the plate becomes thinner. Several plates may be used to increase the stability of the measuring element and to further increase the sensitivity. In order to achieve the same value using a disk shaped plate, it is necessary to use a very large number of discs, which is very expensive. Furthermore, the length of the measuring element is quite large, which represents a further drawback.

  In this arrangement, the housing 7 is on the potential of the first electrode 6a, since this electrode is in direct contact with the housing 7 by one contact surface. The cover 9 includes a metal outer ring 9a which is in direct contact with the housing 7 and also on the potential of the electrode 6a. In the middle, the cover includes a contact site 10 that is insulated from the outer ring 9a and is in electrical contact with the second electrode 6b. This contact is also obtained by a shared contact surface between the electrode 6b and the contact site 10 of the cover 9. In order to prevent a short circuit, the cover is provided with an insulating portion 9b between the outer ring 9a and the central contact site 10. The outer ring 9 a is welded to the housing 7 under pre-tension at the welding site 19.

  If the measuring element is now incorporated in the internal space 14 of the sensor housing 13, the sensor housing must be insulated because the tension sleeve is at the same potential as the first electrode 6a. Therefore, the insulating ring 20 is arranged on both sides of the measuring element with respect to the sensor housing 13. A coaxial cable 15 extends through one of the insulating rings 20 to the cover 9 where it makes electrical contact with the two electrodes 6a, 6b. In such a measurement element, the preamplifier or the line 15 to the amplifier must be composed of a coaxial cable, preferably an additional shield (not shown), in order to isolate it from interference with the measurement signal. .

  FIG. 2a shows in cross section a schematic view of a measuring element with ground insulation according to the invention. Similar to FIG. 1, the same reference numeral is used, and a rod 2 made of a piezoelectric material due to the lateral effect has two opposing faces 3 a, 3 b that serve as surfaces for introducing the force to be measured. The bar 2 further includes two other facing surfaces 4a, 4b provided with metallized surfaces 5a, 5b for collecting surface charges. In this embodiment, both metallized surfaces 5a and 5b extend to electrodes 6a and 6b arranged on the front surface. However, according to the invention of this embodiment, both electrodes 6a and 6b are located on the same front surface 3b. The other front surface 3a is electrically insulated from both electrodes 6a and 6b. This is accomplished, for example, by not extending the metallized surfaces 5a, 5b completely to the front surface 3a as shown on the right side of FIG. 2a. Alternatively, it may be achieved by an edge surface 21 of the front surface 3a arranged after metallization as shown on the left side of FIG. 2a. The front surface 3b now supports two electrodes 6a and 6b. In order to electrically insulate the two from each other, the metallized surface on the front surface 3 b is separated by a separating surface 21.

  Alternatively, the electrodes 6a, 6b can also collect charge directly at the surfaces 4a, 4b for surface charge collection, in which case no front surface needs to be provided with a metallized surface. This can be obtained, for example, by a spring as described in the prior art.

  A housing 7 having an internal space 8 surrounds the rod 2 in this embodiment. The cover 9 encloses the housing 7. The rod 2 is arranged in a pretensioned state between the housing 7 and the cover 9. This is preferably achieved by a welding site 19 between the housing 7 and the cover 9.

  The cover 9 has two identical feed-through pin-shaped contact sites 10a and 10b that are insulated from each other and in electrical contact with the two electrodes 6a and 6b.

  In order to achieve ground insulation, the housing 7 must be electrically isolated from both electrodes 6a, 6b. This is obtained, for example, from the fact that the cover 9 has an external metal ring 9a with an internal insulation 9b, for example made of ceramics, provided with a feed through of two conductors for the contact sites 10a and 10b. Instead of ceramics, other materials suitable for glass or insulators may be selected.

  The bar 2 should be in the center of the housing 7. This can be achieved by providing the housing 7 with a floor plate 7a having a guide groove (not shown) into which the bar 2 is inserted. Conversely, the housing 7 may have one or more openings 11 towards the interior space 8 to provide access to the bar 2 for central positioning that must be performed prior to the application of pretension. These openings 11 can be provided in the floor plate 7a of the housing 7 and / or extend to one or more areas of one or more lateral surfaces. After completion of central positioning, these openings can also be hermetically re-encapsulated.

  In order to simplify the central positioning, a crystal element having a socket-shaped base as described in EP1283552A2 can also be used as a bar. The electrode separation surface must also be arranged accordingly.

  The housing 7 is hermetically sealed. Therefore, if the tension sleeve does not have an opening, the measuring element 1 described here is an already completed sensor, the membrane of which is realized by the floor plate 7a of the housing 7. The two contact sites 10a and 10b on the cover 9 are guided by two conductor cables to the amplifier or preamplifier.

  This arrangement is inventive due to its perfectly symmetrical arrangement with respect to the two poles 22a, b including the contact sites 10a, b to the line with a guide to the metallized surfaces 5a, b, two electrodes 6a, b. In contrast to the arrangement according to the prior art described above, the potential difference between the two lines is evaluated in this arrangement. Therefore, electrical interference from the environment must always have the same effect on both poles of the measurement line, not the difference. For this reason, a coaxial cable cannot be used for transmitting the measurement signal to the preamplifier or the amplifier. This is because external interference signals do not have the same effect on both lines. Therefore, the contact sites 10a and 10b must be the same. These are realized as the same pins, and not as pins and rings as in the prior art.

  In order to compensate for the possible asymmetry of the capacitance, an additional capacitor C may be placed between one pole and the environment, for example the housing 7, which has an appropriate value giving the desired symmetry. .

  The measuring element according to FIG. 2a can already be used as a simple form of sensor, for example as a construction tool.

  There are several possibilities for guiding the electrodes. Figures 2b and 2c show top views of two different embodiments. The embodiment of FIG. 2b corresponds to the display of FIG. 2a, with the surface 21 or other separation on the front surface 3b separating the electrodes 6a, b and the lateral metallized surfaces 5a, b extending to the front surface 3b. Yes. In order to ensure good contact of the metallized surface on the edge between the surfaces 3b to the adjacent surfaces 4a, b, it is desirable to provide rounding 23 on these edges. This is not shown in FIG. 2a. In FIG. 2b, the top view of the bar 2, the positions of these roundings 23 are represented. This type of metallized surface is mainly suitable for thick bars 2.

  In the thinner bar 2, the electrodes are very small in the lateral direction. In this case, another embodiment shown in FIGS. 2c and 2d is desirable. The bar 2 is provided with a rounding 23a at each of the longitudinal edges of one of the surfaces 4a, b for collecting charge and a rounding 23b at the two edges of the front surface 3b not adjacent to the surface 4 for collecting charge. Thereafter, metallized surfaces are provided on all surfaces except the front surface 3a. Finally, surfaces 21a, b and c are provided, separating the metallized surface into two electrically separated electrodes 6a, b. That is, the surface 21a on the front surface 3b in the center between the rounds 23b, the side surface 21b on the front surface 3b so as to obtain the H shape of all the given surfaces, and the surfaces 4a, b for collecting electric charges without rounding Each of the surfaces 21c on the longitudinal edges. The metallized surface described in each of FIGS. 2b and 2c produces two electrically separated electrodes, both of which can be removed on the same front surface 3b. Furthermore, the metallized surface and hence the pole 22 are symmetrical. This new function of lateral effect piezoelectric rods can be used in applications in two-conductor technology.

  FIG. 3 shows in the sensor 12 the sensor housing 13 with an internal space 14, the measuring element 1 mounted in the internal space 14 in the area of the cover 9, and the electrical connection to the two contact sites 10 a, 10 b of the measuring element 1. A sensor 12 is described which includes a membrane 16 having a force transmission surface 17 adjacent to the measuring element 1 together with two conductor cables 15 for connection. Several welding sites 19 hold the sensors 12 together. According to the invention, the measuring element 1 corresponds to the measuring element described in FIG.

  Unlike the measuring element according to FIG. 1, the measuring element 1 is already grounded and can be directly attached to the sensor housing 13 and the membrane 16 without insulation. A so-called anti-strain sleeve 18 can be used, for example, to prevent the transmission of the strain of the sensor housing 13 to the measuring element 1 as occurs by screwing into the part. Instead of attaching the measuring element 1 to the area of the cover 9 by direct connection to the sensor housing 13, as shown in one embodiment on the left side of FIG. A sleeve 18 connects the measuring element 1 in the region of the cover 9 to the sensor housing 13 in the region of the membrane 16. In this way, the deformation of the sensor housing 13 does not exert any force on the measuring element 1 or the bar 2.

  Since the cover 9 is hermetically attached to the housing 7, the space around the rod 2 can be evacuated before welding. This space includes only the internal space 8 of the housing 7 or, if an opening 11 is provided in the housing 7, an anti-strain sleeve 18 between the sensor housing 13 and the measuring element 1 or up to an equivalent connection It is possible to include a part of the inner space 14 of the sensor housing 13.

  In order to increase the stability of the measuring element and at the same time increase the sensitivity, it is also possible to arrange several bars 2 in one housing 7 as illustrated in FIG. For this purpose, it must be ensured that the arrangement is completely symmetrical with respect to the two poles 22. This means that the same number of positive and negative electrode surfaces of the rod 2 must face the outside. Generally speaking, the sum of the positive capacities must be the same as the environment of the rod 2, in this case the negative capacities to the housing 7. Therefore, an even number of bars 2 must be provided. The same polarity electrodes are then mated and connected to the contact site 10 of the two conductor cables.

  Different arrangements of central positioning and pretensioning can be performed if an arrangement such as that illustrated in FIG. 4 is provided having a centrally located device 24 that allows pretensioning. In this case, the central bolt 24 is selected and the pretension sleeve 7 is omitted. In this case, the anti-strain sleeve 18 can be used accordingly.

  The bar 2 includes a metallized surface with electrically different electrodes having capacitive symmetry and includes both electrodes on the same front surface. Instead of a single bar 2, it is also possible to use a block 25 of bars 2.

  The following describes the option of joining several bars 2 to form a block 25. For this purpose, it is desirable, but not essential, to use an odd number of bars 2. As shown in FIG. 5, these rods 2 are each arranged in opposite polarities and are connected to each other by their metallized surfaces 5a, b each having the same polarity. The metallized surfaces 5a, b for collecting charges are attached not only to the contact surfaces but also to the corresponding external surfaces. In order to pass the charge to the front surface 3b, the surface adjacent to the front surface 3b and the charge collection surfaces 4a, b is also metallized. Rounding 23 corresponding to that described in FIG. 2c is valid. The surface 21 provided for separating the metallized surface into two electrically different electrodes corresponds to that of FIG. 2c. In this way, two poles 22a, b can be generated, each extending beyond half of the block 25, as shown in FIG.

  Such a block 25 is more easily centered than a single rod 2 because the rods 2 joined at the metallized surface are combined into a larger block. Further, the manufacture of such a block is advantageous in terms of cost because it can be manufactured by a wafer.

  Blocks 25 consisting of an odd number of bars 2 can be used individually or in groups. By arranging several such blocks so that they are generally symmetrical with respect to their poles, even blocks of bars 2 can be used. An example of the arrangement is shown in FIG. Each of these blocks includes two positive or two negative exterior surfaces. By arranging them in alternating rows, the required symmetry is again obtained.

  An advantage of each of the measuring elements or sensors according to the invention is that grounding insulation is obtained in a simple and therefore cost-effective assembly. Furthermore, there is no need for temperature limitations due to the use of heat sensitive components.

(List of reference numbers)
DESCRIPTION OF SYMBOLS 1 Measuring element 2,2 'Rod 3,3a, 3b Front surface, Surface which introduce | transduces force 4,4a, 4b Surface which collects surface charge 5,5a, 5b Metallized surface 6,6a, 6b Electrode 7 Tension sleeve, Measurement Element housing 7a Tension sleeve / housing floor plate 8 Tension sleeve / housing internal space 9 Cover 9a Outer ring 9b Insulator 10, 10a, 10b Cover contact site 11 Tension sleeve opening 12 Sensor 13 Sensor housing 14 Housing Internal space 15 Cable, line, two conductors or coaxial 16 Membrane 17 Force transmission surface 18, 18a, 18b Anti-strain sleeve having two ends 19 Welding site, Welding 20, 20a, 20b Insulating ring 21 surface, separation surface 22, 22a 22b Pole 23 Rounding 24 Pretensioning element Central bolt 25 block C condenser

In the following, the invention will be described in detail in connection with the drawing.
The schematic representation of the cross section of the sensor by a prior art. 1 is a cross-sectional view of a schematic representation of a measuring element according to the present invention. FIG. 2b is a top view of a piezoelectric rod having a metallized surface as in FIG. 2a. 2b is a top view of the piezoelectric rod of FIG. 2a with different metallized surfaces. FIG. 2c is a cross-sectional view of a piezoelectric rod having a metallized surface as in FIG. 2c. 1 is a cross-sectional view of a schematic representation of a sensor according to the present invention. The figure which shows the symmetrical arrangement | positioning of several bar | burrs in a sensor. FIG. 3 is a top view of a measuring element including several bars arranged on top of each other in an opposing arrangement.

Claims (16)

  In the measuring element (1), two surfaces (3a, b) and two charge collecting surfaces (4a, b) each applying a force to a housing (7) including two poles (22a, b) insulated from each other. ) One or more piezoelectric rods (2) with lateral effects, wherein each pole (22a, b) is insulated from each other and contact site (10a) for connection to a two-conductor measurement signal line (15) B) with one or more electrodes (6a, b) electrically connected thereto and on the charge collection surface (4a, b) of the rod (2) and electrically connected thereto Capacitance defined between each pole (22a, b) and a housing (7) which is electrically insulated according to material properties and geometrical conditions, including metallized surfaces (5a, b) Measuring elements (1), characterized in that they are identical.   2. The measuring element according to claim 1, wherein the poles (22a, b) have a completely symmetrical design with respect to material and geometry.   2. The measuring element according to claim 1, wherein a further capacitor C is provided between one of the poles (22) and the housing (7) to compensate for the difference in the existing capacitance determined by the material or geometry. element.   4. The measuring element according to claim 1, wherein at least one bar (2) has both electrodes (6a, 6b) arranged on the same front face 3b, which are electrically separated from each other. .   5. The measuring element according to claim 1, wherein each of the front surfaces (3a) is electrically insulated from both electrodes (6a, 6b) by one or more rods (2).   6. The measuring element according to claim 1, wherein the housing (7) includes a cover (9) having two conductor feedthroughs.   7. The measuring element according to claim 1, wherein the housing (7) is hermetically sealed.   8. The measuring element according to claim 1, wherein at least one of the bars (2) is pretensioned in the housing (7).   9. The measuring element according to claim 8, wherein the pretension is obtained by a pretensioning sleeve (7), by a membrane or by a central pretensioning element (24).   10. Measuring element according to claim 1, wherein one or more groups of several bars (2, 2 ', 2 ") are arranged symmetrically on the measuring element.   Measuring element according to any of claims 1 to 9, wherein two or more bars (2, 2 ', 2 ") are arranged adjacent to each other in the block (25) in opposite directions with respect to their polarity, Measuring element comprising a metallized surface on the contact surface.   12. Measuring element according to claim 11, wherein one or more groups of several blocks (25) are arranged symmetrically in the measuring element.   13. Measuring element according to claim 10 or 12, wherein a group of bars (2, 2 ', 2 ") or a group of blocks (25) is arranged in one or more rows.   13. The measuring element according to claim 10, wherein the group of bars (2, 2 ', 2 ") or the group of blocks (25) is arranged on a circle.   In the sensor (12), together with a sensor housing (13) having an internal space (14) and a measuring element (1) according to any of the preceding claims attached to the internal space (14), the measuring element (1) A sensor (12) comprising a membrane (16) having an adjacent force transmission surface (17). Sensor according to claim 15, wherein the measuring element (1) is attached to one end (18a) of the anti-strain sleeve (16) in the area of the cover (9) and the other end of the anti-strain sleeve (18). (18b) is a sensor fixed to the sensor housing (13) in the region of the membrane (16).

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